1. Field of the Invention
The invention relates to the field of fluorescence spectroscopy for detecting contaminants in liquid media, and more specifically to a method and a device for measuring the concentration of oil in water.
2. Background of the Invention
In the offshore production of oil, use is made of so-called separation tanks in which the various phases (sand, water, oil and gas) occurring during drilling are separated on the basis of their differences in density and removed in separate conduit systems. In this process, it is possible even for small amounts of oil contaminants in the waste water to lead to intolerable environmental stresses with the corresponding consequences in cost. Typical limiting values on the concentration of oil and water are 40 ppm in the case of direct disposal into the sea, and 100 ppm-5000 ppm in the case of reuse as process water, for example in the case of pumping back into the oil source. In addition, monitoring the concentration of oil in the process water supplies important information on the course of the separation, and serves for monitoring the injection process. This requires oil residue detectors which are sensitive and reliable.
High-pressure separation tanks which are suitable for operation on the sea bed 100 m or more below the surface of the sea have recently been developed. The oil produced and already separated can then be pumped to the surface of the sea with far less expenditure of energy. Such separator tanks are exposed to very high pressures of 60-180 bars, specifically the water pressure at the sea bed from outside, and the pressure of the oil produced on the inside, and to high temperatures of 50-120xc2x0 C. Previously, commercially available detectors cannot be used under these difficult operating conditions. In addition, it is necessary to ensure the oil residue detector can function for years and without maintenance, since an operational failure and premature replacement would entail high costs.
U.S. Pat. No. 5,381,002 presents a fluorescence method for measuring low concentrations of dispersed oil in water. The fluorescence is based on the absorption and frequency-shifted reemission of electromagnetic radiation by aromatic hydrocarbons. The dispersion and solution of the oil in water is improved, inter alia, by mechanical measures such as, for example, by a static mixer, ultrasound or microwaves. UV light sources with exciting wavelengths in the region of 200 nm-400 nm are used, and the fluorescence is detected in the region of 250 nm 600 nm. The fluorescent intensity can be calibrated by means of a reference absorption measurement (xe2x80x9cturbidity meterxe2x80x9d) at the emission wavelength. Non-contaminating free fall measuring cells are also proposed in addition to through-flow measuring cells.
The Sigrist-Photometer AG company markets an oil trace measuring unit which is based on fluorescence excitation by a high-pressure mercury (Hg) vapor lamp. The fluorescent light is detected at 900 to the exciting beam, in order to minimize direct scattered light from the Hg vapor lamp. Measurement is performed by comparison with a reference beam which has a fluorescence standard with a variable light attenuator and is fed through the same Hg vapor lamp. The result overall is an expensive measuring apparatus with a flicker mirror and, possibly, chopper wheel.
It is disadvantageous in the named systems that UV light sources are broadband ones which are of low efficiency, require to be maintained and are relatively short lived. Because of the low fluorescence yield, sensitive photomultipliers are mostly required as detectors. The maintenance interval of an Hg vapor lamp is typically one month, and this calls into question the usefulness of the freedom of other components, in particular a free fall measuring cell, from maintenance. Pulsed operation for the purpose of prolonging the service life is difficult to achieve.
DE 43 05 704 A1 discloses excimer lamps as UV radiation sources for ionizing particles in a gas stream. Excimer lamps supply narrow-band UV radiation by means of a quiet discharge, induced by an E field, in special filling gases. They are distinguished by high stability and long service life, high efficiency and a good degree of suitability for pulsed operation. Consideration is given as filling gases to, for example, inert gases, possibly mixed with metal vapors or halogens and, as the case may be, with a buffer gas, mercury or compounds of such elements. Excimer lamps can be implemented in multifarious forms, for example as flat radiators or as concentric inner or outer radiators.
A description of the design and mode of operation of such UV excimer radiators is also given, inter alia, in the company publication of the applicant xe2x80x9cNeue UV-Strahler fur industrielle Anwendungenxe2x80x9d (xe2x80x9cNew UV radiators for industrial applicationsxe2x80x9d), printed publication CH-E 3.30833.0 D, which is an offprint from the company journal xe2x80x9cABB Technikxe2x80x9d 3/91, pages 21-28.
Accordingly, one object of the invention is to provide an improved fluorescence measuring method for determining the concentration of oil in water, and an improved fluorescence sensor, the method and device being distinguished by simplicity, high exciting efficiency and good long term reliability.
The core of the invention is, specifically, to guide an oil-water mixture through a measuring cell, to use an excimer lamp there to excite fluorescence, to measure an intensity of the fluorescence radiation, and to determine an oil concentration therefrom.
A first exemplary embodiment exhibits a fluorescence measurement with at least one UV excimer flat radiator or tubular radiator, in which measurement fluorescent light is preferably detected at a right angle to the exciting beam and, if appropriate, the excimer power and its attenuation in the measuring cell are monitored.
A second exemplary embodiment represents a modified measuring cell with a concentric UV excimer radiator and, preferably, axially arranged detectors.
Further exemplary embodiments relate to parallel arrangements of an UV excimer tubular radiator and a tubular through-flow cell, an elliptic reflector arrangement and/or a quartz block, resistant to high pressures, for such a configuration, a free fall measuring cell and an installation of an excimer lamp fluorescence sensor for measuring the concentration of oil in water in a high-pressure separator tank.
An important advantage of the fluorescence measurement with excimer lamps includes that, because of the narrow-band, strong excimer radiation, it is possible to implement efficient fluorescence excitation and a high level of detection for dissolved and undissolved oil residues in water, even with photodiodes.
Another advantage includes that the geometry of the excimer lamp can be adapted to the shape of the measuring cell, in order to achieve efficient fluorescence excitation in a large volume, and in order to equip measuring cells with a plurality of redundant excimer radiators.
Further advantages in the use of excimer lamps are their reliability, freedom from maintenance and long service life, particularly in the case of pulsed operation. As a result, it is possible for the first time to implement in-situ fluorescence measurements which are stable in the long term even at locations which are difficult to access.